The present invention relates to a combined feedback control system for hydraulically powered position control servos and, more particularly, to a system in which one or more electro-hydraulic valves may be commanded by an externally generated electrical signal and wherein electric feedback signals are generated to control the output position.
In some instances, it is desirable to provide in a hydraulically powered position control servo an additional stage of power amplification within the servo. To effect such power amplification, the fluid flow from the electro-hydraulic valve (hereinafter referred to as EHV) is used to move or operate a main control valve. The main control valve, then, governs the fluid connections and, thus, the fluid flow to the main ram to control extension and retraction thereof. The main ram may be coupled via a mechanical connection device to an external member to effect position control of the latter as the ram position or displacement from a reference location is changed.
One conventional servo control technique requires two position feedback transducers for such power amplified servo system in order to obtain a stable control system. One of those transducers is coupled to produce a first output signal indicative of the position of the main ram, and the other transducer is coupled to produce a second transducer signal indicative of the position of the main control valve, with such latter position, as is well known, being proportional to the velocity of the main ram. A relatively complex control circuit employs those two transducer signals in separate respective feedback loops to develop a servo control signal which operates the EHV.
Typical electrical feedback loops for fluid servo devices are shown in U.S. Pat. Nos. 2,964,059 and 3,390,613, and mechanical feedback arrangements for fluid devices are shown in U.S. Pat. Nos. 3,270,623, 3,487,750 and 3,540,350. In the 3,540,350 patent, transducers are employed to monitor the operability of certain system portions but are not employed as feedback signal generating devices. Also in the 3,540,350 patent, the main servo valve plunger, which delivers fluid to a ram, is controllably driven by a lever, which is coupled to respond to the electro-hydraulic valve mechanism and to feedback information indicative of the actual ram position. Similarly, in the 3,487,750 and the 3,270,623 patents a lever coupled to the ram delivers mechanical feedback information to a fluid amplifier of an electro-hydraulic valve. In the device of the 3,390,613 patent a lever delivers mechanical feedback information directly to the fluid amplifier at the input of the electro-hydraulic valve and also produces electrical feedback information indicative of the main ram displacement for electrical control of the electro-hydraulic valve.
In the above-mentioned representative prior art patents, the feedback information only represents the ram position and does not relate to the instantaneous ram velocity, which may be an important consideration especially when high inertia loads are encountered. Moreover, in the past conventional control circuitry, which employed plural transducers, such as respective liner variable differential transformers (LVDT) that did consider both displacement and velocity information, the plural feedback channels and relatively complex electronic circuitry required to analyze the feedback and input information were relatively expensive, and with increased complexity found to be less reliable.